Polymer-bound oxidovanadium(IV) and dioxidovanadium(V) complexes: synthesis, characterization and catalytic application for the hydroamination of styrene and vinyl pyridine

Abstract

The Schiff base (Hfsal-aepy) derived from 3-formylsalicylic acid and 2-(2-aminoethyl)pyridine has been covalently bonded to chloromethylated polystyrene cross-linked with 5% divinylbenzene (PS-Hfsal-aepy). Treatment of [V^IVO(acac)₂] with PS-Hfsal-aepy in dimethylformamide (DMF) gave the oxidovanadium(IV) complex PS-[V^IVO(fsal-aepy)(acac)] 1, which on oxidation yielded the dioxidovanadium(V) PS-[V^VO₂(fsal-aepy)] 2 complex. The corresponding neat complexes, [V^IVO(sal-aepy)(acac)] 3 and [V^VO₂(sal-aepy)] 4 have also been prepared. The compounds are characterized in solid state and in solution, namely by spectroscopic techniques (IR, UV-Vis, EPR, ¹H, ¹³C and ⁵¹V NMR), thermal as well as field-emission scanning electron micrograph (FE-SEM) studies. The crystal and molecular structure of [V^IVO(sal-aepy)(acac)] was solved by single-crystal X-ray diffraction. It is a monomeric complex with the tridentate sal-aepy ligand bound equatorially and the two O-atoms of acac⁻ bound at equatorial and axial positions. These complexes catalyze the hydroamination of styrene and vinyl pyridine with amines (aniline and diethylamine) yielding a mixture of two hydroaminated products in good yield. Amongst the two hydroaminated products, the anti-Markovnikov product is favored over the Markovnikov one. Plausible intermediates involved in these catalytic processes are established by UV-Vis, EPR and ⁵¹V NMR studies, and an outline of the mechanism is proposed. The EPR spectrum of the polymer supported V^IVO-complex 1 is characteristic of a magnetically diluted V^IVO-complex, the resolved EPR pattern indicating that the oxidovanadium(IV) centers are well dispersed in the polymer matrix. Neat complexes exhibit lower conversion along with lower turnover frequency as compared to their polymer-anchored analogues. The polymer-anchored heterogeneous catalysts are free from leaching during catalytic action and are recyclable.